Paper sizing refers to the ability of a paper to hold out a liquid or for preventing such liquid from penetrating into or through the paper. Generally the liquid that is held out is water. Compounds that are designed to increase the hold-out of liquids are known as sizing agents. Sometimes a specific type of sizing is referred to, such as an oil sizing agent. For a discussion on sizing see Principles of Wet End Chemistry, by William E. Scott, Tappi Press (1996), Atlanta, ISBN 0-89852-286-2. Sizing values are specific to the test used.
In papermaking and paper finishing, a sizing agent often is employed to provide desirable characteristics sought in the ultimate paper product. Sizing, or sizing property, is a measure of the resistance of a manufactured paper or paperboard product to the penetration or wetting by an aqueous liquid, which may be water. Sizing agents are internal additives employed during papermaking or external additives employed as surface treatment agents during paper finishing that increase this resistance.
Papermaking can be carried out under acidic, neutral, or alkaline pH conditions, and the selection of a sizing agent usually depends upon the pH used. For example, rosin-derived sizing agents typically are used under acidic papermaking conditions. Under alkaline pH conditions, which are widely used in fine paper manufacturing applications, typical sizing agents include alkyl ketene or alkenyl dimers or acid anhydrides, such as alkenyl succinic anhydrides.
A sizing agent may be added to liner board or recycle liner board at the size press on the paper machine. The sizing is often obtained by adding a cationic polymer latex, such as a latex of a polymer of styrene and acrylic monomers. The size press typically contains a dissolved starch, the sizing agent, and other additives. The pH of the size press when the cationic latexes are used is usually between 4.5 and 5.5. At higher pH, the cationic sizing agents are much less efficient at developing sizing. Reactive sizing agents also may be used to size paper, and they are more efficient when the size press pH is above 6.0. Reactive sizing agents are not used extensively for sizing liner board materials, however, because they reduce the coefficient of friction and slide angle of the paper.
Current technology for surface sizing liner board or recycle line board paper relies on application of cationic latex or rosin sizing agents. The efficiency of the sizing is mediocre, and there is room for significant improvement. The sizing is generally conducted at a pH significantly below pH 7, typically at about pH 5.5. Reactive sizing agents are known to provide more efficient sizing when used at the size press in sizing “fine paper,” that is paper for printing and writing applications. However, the use of reactive sizing agents in liner board applications is limited by the deleterious effect such sizing agents have on the coefficient of friction of the final board as noted above.
Accordingly improved methods of sized paper products are desirable in paper making size technology.